The common processing steps involved in making high-quality wafers from the raw material “ingot” are sawing, grinding, lapping, and polishing.
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Bare Wafer Production - No Sawing Without Measuring
The material known as ingot is the first product used in wafer production. It is a block that can be made of silicon or other compound material, for example sapphire or GaAs. A high-precision wafer saw is utilized to cut individual bare wafers. Grooves are made in this procedure, and their depth and width must be monitored.
This is achieved by utilizing optical multi-sensor measuring tools, which not only visualize the saw contour in three dimensions, but also metrologically and quantitatively characterize it. This is a task in which classical optical microscopes cannot be of benefit because of a lack of height information.
The sawing process can be greater controlled due to quantitative process control which means that the specification deviations of the sawn wafers are considerably lower. Additionally, the behavior of various materials throughout the sawing process can be recorded and the tool wear of the processing machines can be examined.
Figure 1. Area measurement of saw marks on a Si wafer
Figure 2. Topography of a Si wafer along the blue line
Quality Standards of the Producers are Written with Capital Letters
Manufacturers in the fields of microsystems technology, microelectronics and photovoltaics have great demands on the manufacturing capabilities of the main product ‘wafer’, as even slight deviations can negatively influence the quality in the cost-intensive, downstream process steps.
The outcome of this is a decrease in yield, along with less efficiency and reliability in the end products. High-quality, partly automatic multi-sensor measuring technology benefits the monitoring of process tolerances in wafer processing and enables the necessary quality standards of the producers to be maintained.
Process Control for Wafer Thinning and Polishing
Post-sawing, the wafers are processed further by employing mechanical processes for example polishing, lapping, and grinding. A thickness in the range of a few micrometers must be maintained throughout the entire wafer.
As the wafer is frequently doped with a predetermined doping depth, the grinding procedure must be tailored to ensure that the thickness does not decline below a specific level. This is particularly true for thin wafers with lower tolerances because the necessity for reproducibility and accuracy increase.
Thickness Deviation is not Desired - TTV AND CO.
Optical non-contact thickness measurement has established itself in this respect. The same technology is also utilized to specify the roughness with a high resolution. Thin, taped or bonded wafers can also be evaluated.
FRT is reliant upon a diametrically arranged, calibrated sensor configuration containing two non-contact chromatic point sensors for surface measurement and wafer thickness. They record the distance to the wafer at its top and bottom.
According to the SEMI standard, the local wafer thickness along with the thickness variation is accurately determined throughout the total surface of the wafer. Specifically, the thickness variation, which is defined by the TTV value, is a sign of quality in wafer grinding.
It enables decisions to be made about whether the material has been uniformly removed. Complicated tasks can only be carried out with wafers with a very low TTV value.
Figure 3. Full wafer thickness map in 3D view, polished Si wafer
MicroProf® FE - Popular in Every Front-End HVM FAB
A MicroProf® FE is now available to view in the FRT clean room. Come and visit to experience the MicroProf® FE live.
The MicroProf® FE has been created for the automated measurement of local and global wafer parameters such as bow, TTV, roughness, warp, and waviness, for the quantitative determination of defects resulting from mechanical processing steps and the artifact-free characterization of the nanotopography of 300 mm wafers.
Figure 4. Wafer map with local parameters (LTIR, LTV, LT, LFPD, etc.)
Figure 5. Full wafer map of a Si wafer in 3D view showing bow
Figure 6. 3D topography of a Si wafer with voids
It brings together the features of the globally established MicroProf® 300 with a wafer handling system within an Equipment Front End Module (EFEM) and is supplied with filter units (FFU) to satisfy the ISO Class 3 clean room requirements.
The handling unit has two load ports comprising of a mapper and RFID reader, along with a pre-aligner and a single-arm robot with a vacuum end effector or an edge grabber. The MicroProf® FE is designed to enable fully automated measurement with 300 mm FOUPs/FOSBs.
Additionally, the tool can be calibrated to process open cassettes (150 mm / 200 mm) and exclusively for 200 mm or 300 mm wafers, or as a 200 mm / 300 mm bridge tool. Along with the standard configuration, several additional functions can be added to the tool, which can also be retrofitted at a later stage.
Handling can also be carried out for non-standard wafers such as highly warped wafers (for example eWLB), or thin wafers down to 50 μm in thickness. Similarly, more sensors can be retrofitted at a later date due to the multi-sensor concept.
Owing to its fully SEMI-compliant measurement solutions, almost maintenance-free hardware components and its high throughput, the MicroProf® FE is the ideal workhorse in any front-end HVM-Fab. With a decline in investment costs, it is always possible to adhere to the advancing developments in technology.
Make an appointment with FRT to see the MicroProf® FE live.
For more information, please click here.
For additional comments or questions, please contact FRT where the experts will be happy to provide solutions to measuring tasks.
This information has been sourced, reviewed and adapted from materials provided by FRT Metrology.
For more information on this source, please visit FRT Metrology.